Amanti et. al. Low-divergence single-mode terahertz quantum cascade laser, Nature Photonics, Vol. 3, page 586 (Published 2009) discloses a single mode semiconductor terahertz laser source with symmetric beam patterns. However, the mode selectivity of Amanti et al. is poor and single frequency is not always guaranteed. There is a need for better frequency control.
Amanti et. al. Low divergence Terahertz photonic-wire laser, Optics Express, Vol. 18, Issue 6, page 6390 (Published 2010) discloses a single mode semiconductor wire terahertz laser source with symmetric beam patterns. The total device length (and thus the total radiation power) is limited due to the phase mismatch phenomenon and the maximum electric current a wire laser can sustain. The device power out-coupling ability is also governed by its given geometry and cannot be adjusted. There is a need for better electric biasing mechanism. There is a need for adjustable power out-coupling.
Solbach et. al. Microstrip-Franklin Antenna, IEEE Transactions on Antennas and Propagation, Vol. 30, Issue 4, page 773 (Published July 1982) discloses a type of microstrip rectangular patch arrays with phasing stubs to achieve a co-phase current distribution driven by external sources.
Terahertz QCLs are expected to find applications as local oscillators for sub-millimeter wave heterodyne receiver systems at frequencies above 1 THz. Their low power consumption and small form factors make them suitable for applications in astrophysics and remote sensing. A robust single-mode laser that can sustain continuous-wave (cw) operation at an operating temperature of >77 K and emit >1 mW of optical power in a narrow and symmetric beam pattern is highly desired in these applications.
The best terahertz QCLs in terms of high-temperature operation have been demonstrated based on the metal-metal (MM) waveguides, which provide strong mode confinement and low waveguide losses. However, due to the sub-wavelength confinement of the waveguide, far-field beam pattern of a Fabry-Perot (F-P) MM THz laser is far from ideal Gaussian beams. Several approaches have been demonstrated to address this issue, such as photonic crystals or surface-emitting DFB laser arrays. The majority of these approaches focus on enlarging the two-dimensional light emitting area and thus suffer from degradation in continuous-wave (cw) performance due to their large power dissipation and difficulty in heat removal. Recently, a solution based on third-order DFB structures (Amanti et. al.) utilizes the so-called end-fire antenna effect to achieve tight beam patterns while minimizing the negative impact on cw performance.